Power Transformer Question
Power Transformer Question
(OP)
I am working on a Power Distribution Unit where the ETL Listed Equipment has a 500A Main CB feeding
a 300kVA Delta-Wye Transformer (480: 208/120). The Transformer Secondary Feeds Six 225 Amp Molded Case
Circuit Breakers (ie. Primary Protection Only). What NEC Sections would pertain to the sizing and length
of the secondary conductors feeding the six circuit breakers on the secondary side? I am questioning the
size of the conductors used.
a 300kVA Delta-Wye Transformer (480: 208/120). The Transformer Secondary Feeds Six 225 Amp Molded Case
Circuit Breakers (ie. Primary Protection Only). What NEC Sections would pertain to the sizing and length
of the secondary conductors feeding the six circuit breakers on the secondary side? I am questioning the
size of the conductors used.






RE: Power Transformer Question
RE: Power Transformer Question
Based upon condition of acceptability 240.21 (C)(2)(1),
I am still confused by the requirement for the secdondary
conductor ampacity to be not less than the combined calculated
loads AND not less than the rating of the device supplied by the
secondary conductors OR not less than the rating of the Overcurrent
Protective Device at the termination of the secondary conductors.
The THHN conductors maintain separation and the manufacturer is using NEC
table 310.17 for the ampacity calculations (75 Deg C). The wires terminate
into an 80% rated 225 Amp MCCB, so the ampacity being used is 180 Amps
(80% or 225). Does this jive with the later half of 240.21 (C)(2)(1),
or do the conductors need to be sized for the full 225 Amps? I think the intent
of the code is what I am missing.
RE: Power Transformer Question
(If you have already taken 125% of the continuous load for the sizing calculation, you do not need to apply the 80% de-rating. The 125% and 80% factors are two sides of the same coin. 1/1.25 = 0.8)
RE: Power Transformer Question
conductor sizing for these applications. But I did not design it
and I was hoping to site the NEC for a clear definition on this.
The calculated load is less than 180 Amps, the MCCB is correctly
sized at 225 Amps, the unprotected transformer secondary conductors
feeding the 225A MCCB are #1/0 AWG THHN CU. Using 310.17 (or 310.15(B)(17)
in the 2011 NEC) the 75 DEG Ampacity is 230 Amps at 30 Deg C.
Based upon your comments, this is sound as long as there are no
correction factors required for temperature.
When would the NEC condition: "not less than the rating of the
device supplied by the secondary conductors OR..." apply? Why is this
statement necessary if only the OCPD rating is required? I cannot
think of an application where this would apply.
RE: Power Transformer Question
"Device" in the NEC is not the same as an "Overcurrent Protecive Device"
From the NFPA commentary:
RE: Power Transformer Question
pertain to a situation where the secondary of a transformer is tapped to feed 2 ea.
Main Lug 225 Amp Rated Load Centers, then the wire would need to be sized per the full
"rating" of the device (ie. 225 Amps) as there would be no OCPD until the branch circuit level.
There is no typo on the 1/0 AWG, this comes from table 310.17 in the 75 Deg C column for
ampacity in free air. (The conductors are not in conduit).
RE: Power Transformer Question
I have an old GE publication, GET-2779J. I assume something similar is still available.